WASHINGTON -- Senate Republicans' rejection of a U.N. treaty on disability discrimination earlier this month won headlines, but a similar blow last July to the U.N. Arms Trade Treaty drew mostly yawns.

That may have been because once the National Rifle Association weighed in against the arms-trade measure, its fate became a foregone conclusion.

Warning the pact would put "international controls" on guns and "undermine the constitutional rights of law-abiding American gun owners," the NRA dialed up opposition from 4 million-plus members, who flooded House and Senate offices with emails, letters and phone calls.

U.N. negotiators insisted the treaty was aimed at terrorists and drug lords, and would not impose any restrictions on civilian gun owners whatsoever.

Nonetheless, without U.S. support, they shelved it.

It was the latest in a line of NRA political victories dating to at least 1934, when it succeeded in watering down the National Firearms Act, approved by Congress in response to Prohibition-era gun violence.

Its virtually unrivaled winning streak on Capitol Hill since ranges from opposition to armor-piercing bullets in the 1980s to a more recent drive to emasculate the Bureau of Alcohol, Tobacco, Firearms and Explosives, the federal agency charged with enforcing federal firearms laws.

"They are a very powerful organization," said former Rep. Charlie Stenholm, D-Texas, a staunch gun-rights supporter who tangled with the NRA a few times up to his defeat in 2004, when the group endorsed his Republican rival.

President Barack Obama, and Democrats on Capitol Hill now are responding to growing public sentiment in favor of gun control in the wake of the Newtown massacre, in which a Bushmaster .223 AR-15 semiautomatic rifle was used to slay 20 children and six adults at Sandy Hook Elementary.

The NRA hunkered down in "no comment" mode for a week after the shootings, before its executive vice president Wayne LaPierre put forward a plan to place armed volunteer guards in schools and was met with near-universal scorn among politicians and the media. Earlier last week, one such group, Gun Owners of America, contrasted its belief that designated gun-free zones made schools a tempting target with NRA silence on the issue.

"One of the great untold stories about the NRA is that it's in a constant battle with grassroots members," said Ross Baker, a Rutgers University political science professor who says he is a gun owner and on-again, off-again NRA member. "There is a feeling that NRA lobbyists in Washington have gone native, that they're more interested in making deals than protecting gun rights."

The NRA fought gun-buyer background checks, but supported them once they became computerized and gun purchasers waited minutes rather than days for the FBI system to approve or deny them.

In 2007, the NRA also signed off on improving the background checks' ability to flag prospective purchasers disqualified by mental illness -- a measure opposed by Gun Owners of America.

The shootings capped a year of mixed results for the NRA. In the 2012 election, the organization saw its investment of more than $18 million against Obama and Democratic Senate nominees go down in flames.

Still, the NRA has long been able to turn on and turn off political heat in the halls of Congress, state capitols or local town councils. Veteran observers of the Washington influence game warn against discounting the gun group's clout.

"On the Hill, the tradition is if you cross the NRA, there's a price to pay," said Josh Sugarmann, a Newtown native and gun-control advocate who is executive director of the Violence Policy Center and author of a book "NRA: Money, Firepower and Fear."

"You cross victims of gun violence, there's been no price to pay. Usually there's no balance between the NRA and Americans who favor gun control. That balance, unfortunately, only arises in the aftermath of shootings like those in Newtown."